The present invention relates to a reproducing signal measuring method for optical recording medium with due regard to interlayer crosstalk of the optical recording medium having a plurality of recording layers, and a signal reproducing apparatus and an optical recording medium having a plurality of recording layers which realize the reproducing signal measuring method.
FIG. 1 shows a sectional structure of a conventionally known multilayer optical disk among optical recording media. A principle for selectively recording and reproducing information of each recording layer is schematically shown in FIG. 1. In the conventional art, a recording medium includes a total of six layers having information recorded thereon (hereafter referred to as recording layers). The recording medium includes a first recording layer 101, a second recording layer 102, a third recording layer 103, a fourth recording layer 104, a fifth recording layer 105 and a sixth recording layer 106 in the cited order beginning with an opposite side from the light incidence on the optical disk, i.e., from the bottom side of FIG. 1. For accessing recorded information, for example, on the third recording layer 103 by using the six-layer medium having six recording layers, the position of an object lens in an optical recording and reproducing apparatus is controlled and an optical spot 107 is positioned on the third recording layer 103. At that time, converging rays 108 in the middle of being narrowed down by the object lens are transmitted by the sixth recording layer 106, the fifth recording layer 105 and the fourth recording layer 104 which are semi-transparent. On the sixth, fifth and fourth recording layers, the luminous flux diameter of the converging rays 108 is sufficiently larger than the diameter of the optical spot 107 on the third recording layer 103. Therefore, recording information on the sixth, fifth, fourth recording layers 106, 105 and 104 which are semi-transparent cannot be resolved and reproduced. Since the luminous flux diameter is large on the sixth, fifth, fourth recording layers 106, 105 and 104 which are semi-transparent, the light intensity per unit area becomes relatively small. Accordingly, there is no fear of destroying information on the sixth, fifth, fourth recording layers 106, 105 and 104 at the time of recording. In this way, information recording and reproducing on the third recording layer located remote from the incidence side of the irradiation light are implemented without being influenced by the sixth, fifth, fourth recording layers. Information recording and reproducing on other recording layers are conducted in the same way by controlling the position of the object lens. Conditions for thus conducting recording and reproducing on an optical recording medium having a plurality of recording layers without exerting influence upon other layers are described in detail in JP-A-5-101398.
In the above-described multilayer disk reproducing method, it is considered that the effect of light attenuation is given by a recording layer located on the irradiation light incidence side of a target recording layer, however, it is not considered that an influence is caused by a phenomenon of multiple reflection of light in a recording layer located on this side of the target recording layer for the recording and reproducing is not taken into consideration. The state in which multiple-reflected light poses a problem in information recording and reproducing will now be described with reference to FIG. 4. It is now supposed that the target recording layer of recording and reproducing is an nth layer. Incident light 108 formed of converging rays is applied so as to form an optical spot 107 on the nth layer. At this time, light reflected by an (n+1)st layer located on this side of the target recording layer becomes unnecessary light 402. After arriving at back of an (n+2)nd layer and being reflected by the back of the (n+2)nd layer, the unnecessary light 402 might be reflected by the (n+1)st layer again, follow nearly the same path of light reflected by the nth layer, and be detected as if it is light reflected by the nth layer. In this case, large interlayer crosstalk is generated. In the present specifications, the “crosstalk” means “interlayer crosstalk.” It is pointed out in Japanese Journal of Applied Physics, Vol. 43, No. 7B, 2004, pp. 4983-4986 and Ushiyama et. Al., Tech. Digest of ODS2006, WDPDP3 that detection of such unnecessary light poses a great problem.